This invention relates to a hydrogen getter capable of preventing a cladding tube from becoming brittle and being destroyed by hydrogen gas and also to a nuclear fuel element provided with said hydrogen getter.
A nuclear fuel element used with various types of nuclear reactor generally consists of fuel material sealed in a container or cladding tube made of corrosion-resistant, nonreactive and heat-conducting material. The cladding tube is customarily formed of stainless steel, aluminium or alloys thereof, or zirconium or alloys thereof. However, a cladding tube constructed of, for example, zirconium or alloys thereof sometimes tends to get locally hydrogenated and brittle by hydrogen gas evolving from various sources in the cladding tube during the run of a nuclear reactor and be destroyed by the resultant increase in a pressure difference originally occuring inside and outside of the cladding tube.
A nuclear fuel element used with, for example, a light-water type power reactor is constructed by packing a zircalloy cladding tube with pellets of uranium dioxide as fuel material and sealing helium gas in said cladding tube. The uranium dioxide pellets generally contain hydrogen and remnants of hydrogen atom-carrying organic compounds, for example, metal salts of stearic acid used as a binder. All these contaminants are generally liable to be brought into said pellets while they are prepared. Further, water particles tend to be adsorbed to the surface of the uranium dioxide pellets or the inner wall of the cladding tube. Consequently, hydrogen gas, water vapor and hydrocarbon gas are generated in the tightly closed nuclear fuel element while a nuclear reactor is operated. Water vapor and hydrocarbon gas often release hydrogen by reaction with the cladding tube and uranium dioxide pellets. No serious problems arise, as long as atmosphere in the nuclear fuel element containing the above-mentioned gasseous mixture is of the oxidizing type. Where, however, said atmosphere is turned into the reducing type due to increased amounts of released hydrogen, then the zircalloy cladding tube is rapidly hydrogenated and becomes brittle. FIG. 1 sets forth the measured hydrogen-absorbing property of zircalloy in an atmosphere (1 atmospheric pressure) consisting of a water vapor-hydrogen mixture with the proportions of both gases varied during experiments. In FIG. 1, a ratio of the partial pressure of hydrogen to that of water vapor is plotted on the abscissa, and an amount of hydrogen absorbed in 6 hours after the start of the experiment is plotted on the ordinate, both plottings being given in terms of log graduations. As apparent from FIG. 1, the more prominent the reducing tendency of the atomsphere, and the higher the atmosphere temperature, the larger the amount of hydrogen absorbed by zircalloy, resulting in the more accelerated brittleness thereof. It will therefore be easily understood that a nuclear fuel element is quickly hydrogenated in a high temperature reducing atmosphere easily produced in the cladding tube during the run of a nuclear reactor.
As already known, a hydrogen getter alloy for absorbing hydrogen gas and water vapor has been used to prevent the cladding tube from being hydrogenated and becoming brittle by hydrogen gas. A hydrogen getter alloy known to-date is, for example, a nickel-titanium-zirconium hydrogen getter alloy (hereinafter referred to as a "Ni-Ti-Zr getter alloy") marketed by General Electric Company, U.S.A. This Ni-Ti-Zr getter alloy is formed of about 3 to 12 wt% of nickel, about 3 to 30 wt% of titanium and zirconium as the remainder. However, this type of hydrogen getter alloy has the drawback that the alloy material reacts with water vapor in the hydrogen-water vapor atmosphere to have the surface coated with a protective film, resulting in a decline in the hydrogen-absorbing capacity of said getter alloy. Generally, vapors are actually evolved from the water adsorbed to the surface of uranium dioxide pellets and the inner wall of a cladding tube ahead of hydrogen gas, causing a getter alloy to absorb water and have the surface coated with a protective film due to reaction between both materials. As the result, the getter alloy not only decreases in the hydrogen-absorbing capacity, but also renders the atmosphere of the cladding tube more of the reducing type, thereby giving rise to the danger of the cladding tube itself being hydrogenated.
Another hydrogen-removing process already applied is to weld an end plug to a cladding tube, seal helium gas in the cladding tube after evacuating it and dry off absorbed water by applying high temperature during the evacuation. However, this process still has the drawbacks that the cladding tube has to be quickly sealed after drying; the sealed atmosphere of the cladding tube is rendered more of the reducing type due to the subsequent generation of various gases in the cladding tube, failing to minimize the danger of said cladding tube being again hydrogenated; and in consequence said prior art process is unadapted for permanent applicability.